1. Field of the Invention
This invention relates to a body detecting device which detects a body by using a transmitter and a receiver, and more particularly to a body detecting device which can always perform a stable detecting function for any fluctuation of reception signal resulting from any other factor than the body.
In a device which uses a set of transmitter and receiver for signal media propagated through the space such as electromagnetic waves (including light waves), sound waves or the like, to detect the influence imparted from a body to the reception signal to thereby detect the presence, condition and position of the body, the reception signal, especially the reception gain, may usually be fluctuated by the atmospheric conditions such as temperature, humidity, and mixture of special gases with the air, and may further be fluctuated, even under the same atmospheric conditions, by unstable factors such as contamination of the transmitter and/or the receiver, irregularities of their efficiencies, difference in arrangement or installation of the transmitter and the receiver into the detecting device, and variation in efficiency with time.
These fluctuations are caused entirely independently of the condition of presence or position of the body and therefore, increase the detection error, which means a great drawback when the device of this type is used.
2. Description of the Prior Art
In order to overcome these disadvantages, it has heretofore been the practice either to use a detecting device provided with a set of transmitter and receiver hardly susceptible to the influences of the above-described variable factors, or to calibrate the gain of the transmitter and receiver each time the device is used or to use the device only under limited conditions, whereas in these cases the detecting device is expensive or the conditions for its use are limited. For example, Japanese Patent No. 58866 published in 1973 discloses a device provided with a transmitter and a receiver wherein reception gain is fed back to the transmitter side to correct any fluctuation of the reception gain, but this is unsuitable for the detection of a transparent body because it is of the type in which the brightness of the light source employed as the transmitter is corrected in accordance with a specific fluctuation of the quantity of light received, and accordingly the types of the body to be detected by such device are limited and moreover, the arrangement and locations of the transmitter and the receiver are also limited inasmuch as feedback between the transmitter and the receiver is utilized.